To define an intellectual disability, 42 CFR 483.102(b)(3)(i) incorporates by reference the definition in AAMD's 1983 Manual. Copies of that manual are hard to find. An scanned copy is provided here. Page 11 indicates that a "developmental period" ends at age 18.
These slides provide written CMS guidance clarifying the definition of "Specialized Services" as the term is used in the PASRR program. These slides were first presented at the 2013 Annual Conference on Home and Community-Based Services.
Full text of the Americans with Disabilities Act (originally enacted in 1990), which laid the groundwork for the Supreme Court decision Olmstead v. L.C.